mtr does not restart the server whose parameters were changed

If a mtr test runs multiple servers and only some of them get
restarted on whatever reason with new command-line parameters,
then subsequent mtr test may fail, because no cleanup is performed.
Replication and Galera test suites are affected.

In the mtr script, there is a server_need_restart function
that decides whether we need to start a new mysqld process before
the new (next) test. If the mysqld parameters were changed in the
previous test - not necessarily the parameters of the primary mysqld
server, maybe even the secondary server parameters - this function
decides to start a new mysqld process. But since it does not remove
the old (changed) parameters, the new process starts with the
parameters changed by the previous test.